Process for preparing ceramic materials

ABSTRACT

A method of preparing ceramic materials or articles by the pyrolysis of preceramic silazane polymers, wherein the preceramic silazane polymers are rendered infusible prior to pyrolysis by treatment with steam or a steam and oxygen mixture, is disclosed. This method is especially suited for the preparation of ceramic fibers.

The Government has rights in this invention pursuant to contract No.F33615-83-C-5006 awarded by the U.S. Air Force.

BACKGROUND OF INVENTION

This application is a continuation of U.S. application Ser. No. 868,177,filed May 23, 1986, U.S. Pat. No. 4,631,260, which is a continuationapplication of U.S. application Ser. No. 748,109 filed June 24, 1985,now abandoned.

This invention relates to the preparation of ceramic materials orarticles by the pyrolysis of preceramic silazane polymers wherein thepreceramic silazane polymers are rendered infusible prior to pyrolysisby treatment with steam or a steam and oxygen mixture. This method isespecially suited for the preparation of ceramic fibers.

The prior art discloses that ceramic materials have been prepared by thepyrolysis of preceramic silazane polymers. Gaul in U.S. Pat. No.4,312,970 (issued Jan. 26, 1982) obtained ceramic materials by thepyrolysis of preceramic silazane polymers, which polymers were preparedby reacting organochlorosilanes and disilazanes. The preceramic silazanepolymers were pyrolyzed in an inert atmosphere without any separatetreatment to render the silazane preceramic polymer infusible.

Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982) obtained ceramicmaterials by the pyrolysis of preceramic silazane polymers, whichpolymers were prepared by reacting chlorine-containing disilanes anddisilazanes. Fibers prepared from such preceramic silazane polymers weregiven a "mild heat treatment" in air before pyrolysis but there is noteaching that such a treatment rendered the fibers infusible.

Cannady in U.S. patent application Ser. No. 555,755, filed Nov. 28,1983, now U.S. Pat. No. 4,540,803, obtained ceramic materials by thepyrolysis of preceramic silazane polymers, which polymers were preparedby reacting trichlorosilane and disilazane. The preceramic silazanepolymers were not rendered infusible prior to pyrolysis, in order toform ceramic materials.

What has been discovered is a method of rendering preceramic silazanepolymers infusible prior to pyrolysis. This method represents asignificant advance in the art of preparing ceramic materials orarticles, especially in the art of preparing ceramic fibers.

THE INVENTION

This invention relates to a method of preparing a ceramic material,which method comprises (1) treating a preceramic silazane polymer withsteam for a time sufficient to render the preceramic silazane polymerinfusible wherein the treatment temperature is sufficiently low so thatthe preceramic silazane polymer remains unfused during the treatmentstep and (2) heating the infusible preceramic silazane polymer of step(1) in an inert atmosphere, vacuum or ammonia-containing atmosphere to atemperature of at least 750° C. until said infusible preceramic silazanepolymer is converted to a ceramic material.

This invention also relates to a method of preparing a ceramic material,which method comprises (1) treating a preceramic silazane polymer withsteam at a temperature below the penetration temperature of thepreceramic silazane polymer for a time sufficient to render thepreceramic silazane polymer infusible and (2) heating the infusiblepreceramic silazane polymer of step (1) in an inert atmosphere, vacuumor ammonia-containing atmosphere to a temperature of at least 750° C.until said infusible preceramic silazane polymer is converted to aceramic material.

This invention also relates to a method of preparing a ceramic material,which method comprises (1) treating a preceramic silazane polymer with asteam and oxygen mixture for a time sufficient to render the preceramicsilazane polymer infusible wherein the treatment temperature issufficiently low so that the preceramic silazane polymer remains unfusedduring the treatment step and (2) heating the infusible preceramicsilazane polymer of step (1) in an inert atmosphere, vacuum orammonia-containing atmosphere to a temperature of at least 750° C. untilsaid infusible preceramic silazane polymer is converted to a ceramicmaterial.

This invention also relates to a method of preparing a ceramic material,which method comprises (1) treating a preceramic silazane polymer with asteam and oxygen mixture at a temperature below the penetrationtemperature of the preceramic silazane polymer for a time sufficient torender the preceramic silazane polymer infusible and (2) heating theinfusible preceramic silazane polymer of step (1) in an inertatmosphere, vacuum or ammonia-containing atmosphere to a temperature ofat least 750° C. until said infusible preceramic silazane polymer isconverted to a ceramic material.

This invention further relates to ceramic fibers prepared by the methodcomprising the steps of (1) preparing a preceramic silazane polymer, (2)preparing preceramic fibers from said preceramic silazane polymer, (3)treating the preceramic fibers prepared in step (2) with steam or asteam and oxygen mixture for a time sufficient to render the preceramicfibers infusible wherein the treatment temperature is sufficiently lowso that the preceramic silazane fibers remain unfused during thetreatment step and (4) heating the infusible preceramic fibers of step(3) in an inert atmosphere, vacuum or ammonia-containing atmosphere to atemperature of at least 750° C. until said infusible preceramic fibersare converted to ceramic fibers.

This invention also relates to ceramic fibers prepared by the methodcomprising the steps of (1) preparing a preceramic silazane polymer, (2)preparing preceramic fibers from said preceramic silazane polymer, (3)treating the preceramic fibers prepared in step (2) with steam or asteam and oxygen mixture at a temperature below the penetrationtemperature of the preceramic fibers for a time sufficient to render thepreceramic fibers infusible and (4) heating the infusible preceramicfibers of step (3) in an inert atmosphere, vacuum or ammonia-containingatmosphere to a temperature of at least 750° C. until said infusiblepreceramic fibers are converted to ceramic fibers.

Passage of steam or a steam and oxygen mixture over a preceramicsilazane polymer will result in an infusible preceramic silazane polymersuitable for pyrolysis to form a ceramic material. Preceramic silazanepolymers in the form of pellets, powders, flakes, foams, fibers, and thelike are especially suitable for treatment with steam or a steam andoxygen mixture by the method of this invention. The pressure of thesteam or the steam and oxygen mixture is not critical. High pressuresteam may be used. It is preferred, however, that pressures at or nearatmospheric pressure be used for ease of operation. The steam or steamand oxygen mixture may, if desired, contain inert gas dilutents such asargon, nitrogen and the like. The oxygen in the steam and oxygen mixturemay be pure or relatively pure oxygen or may be in the form of air. By"steam" we mean both essentially 100 weight percent water vapor andwater vapor in an inert carrier gas such as nitrogen, argon, helium,carbon dioxide and the like.

The temperature of the treatment of the preceramic silazane with thesteam or steam and oxygen mixture must be sufficiently low so that thepreceramic polymer does not melt or fuse during the treatment step.Preferably the treatment temperature is below the penetrationtemperature or softening temperature of the preceramic silazane polymer.The treatment temperature must be above the temperature at which thesteam will condense as water droplets on the preceramic silazane polymerbeing treated. As one skilled in the art would realize, the penetrationtemperature of individual preceramic silazane polymers will depend inlarge part upon the reactants and reaction conditions employed toprepare each preceramic silazane polymers. Therefore the penetrationtemperature of a given preceramic silazane polymer should be determinedto establish the preferred upper temperature limit of treatment withsteam or a steam and oxygen mixture. More preferably the temperature ofthe treatment with steam or a steam and oxygen mixture should be betweenabout 35° C. and about 10° C. below the penetration temperature of thepreceramic silazane polymer. The treatment temperature must be, however,above the temperature at which steam will condense as water droplets onthe preceramic silazane polymer being treated. The pressure of thetreatment gas can be varied to help control condensation. Shortertreatment times to render the preceramic silazane polymer infusible canbe expected if the treatment temperature is kept as high as possible.Within these constraints, it generally has been found that temperaturesbetween about 35° and 200° C. are suitable. One way to avoid thepossibility of water condensing on the ceramic article during treatmentis to use air or oxygen saturated with water vapor at room temperatureand a treatment temperature greater than about 10° C. above roomtemperature.

The preceramic silazane polymers are treated with steam or a steam andoxygen mixture for a time sufficient to render the preceramic silazanepolymer infusible. What is meant by "infusible" in this specification isthat the treated preceramic silazane polymer when heated rapidly up tothe pyrolysis temperature will not fuse together. A crude screen forinfusibility is provided by the solubility of the preceramic silazanepolymer in toluene. Prior to treatment with steam or a steam and oxygenmixture the preceramic silazane polymers are almost completely solublein toluene. The infusible preceramic silazane polymers obtained bytreatment by the method of this invention are either insoluble intoluene or have only limited solubility in toluene. The time required torender the preceramic silazane polymer infusible by the method of thisinvention will depend, in part, on the size of the preceramic silazanepolymer object, the temperature of the treatment, the amount of watervapor or water vapor and oxygen present, and the specific preceramicsilazane polymer employed. The time required to render the preceramicsilazane polymer infusible will normally be in the range of a fewminutes to several hours or longer. It is best to determine thetreatment time by routine experimentation.

The amount of steam or steam and oxygen mixture that the preceramicsilazane polymer should be exposed to is the amount sufficient to renderthe preceramic silazane polymer infusible. This required amount willvary from case to case depending, in part, upon the temperature,pressure, the time of exposure and the actual preceramic silazanepolymer used as well as other variables. When the preceramic silazanepolymer is in the shape of a formed object such as a fiber it is notnecessary to render the entire shaped article infusible. Rather only theouter surfaces, and sufficient interior portions directly adjacent tothe outer surfaces, need be rendered infusible. The interior portion ofthe shaped article may be cured during the pyrolysis of the shapedarticle to elevated temperature. Simply rendering the exterior infusiblewill prevent the shaped articles from fusing together during thepyrolysis unless a break in the exterior surface occurs which allows thenonfused interior to leak out.

Preceramic silazane polymers suitable for use in this present inventionare well known in the art. The preceramic silazane polymers suitable foruse in this invention must be capable of being converted to a ceramicmaterial at elevated temperatures. It is generally preferred that thepreceramic silazane polymers used in this invention be capable of beingconverted to a ceramic material in at least 40 weight percent yield.Mixtures of preceramic silazane polymers may also be used in thisinvention. Examples of preceramic silazane polymers or polysilazanessuitable for use in this invention include polysilazanes as described byGaul in U.S. Pat. Nos. 4,312,970 (issued Jan. 26, 1982), 4,340,619(issued July 20, 1982), 4,395,460 (issued July 26, 1983), and 4,404,153(issued Sept. 13, 1983), all of which are hereby incorporated byreference. Suitable polysilazanes also include those described byHaluska in U.S. Pat. No. 4,482,689 (issued Nov. 13, 1984) and bySeyferth et al. in U.S. Pat. No. 4,397,828 (issued Aug. 9, 1983), bothof which are hereby incorporated by reference. Other polysilazanessuitable for use in this invention are disclosed by Cannady in U.S.patent applications Ser. No. 555,755 (filed Nov. 28, 1983), Ser. No.627,260 (filed July 2, 1984), now U.S. Pat. No. 4,535,007, and Ser. No.689,258 (filed Jan. 7, 1985), now U.S. Pat. No. 4,543,344, by Bujalskiin U.S. patent application Ser. No. 653,003 (filed Sept. 21, 1984), nowabandoned, and by Baney et al. in U.S. patent applications Ser. No.652,938 (filed Sept. 21, 1984), now abandoned, and Ser. No. 653,939(filed Sept. 21, 1984), now abandoned, all of which are herebyincorporated by reference. Still other polysilazanes may be suitable foruse in this invention.

Preceramic silazane polymers especially useful in this invention aredescribed in U.S. Pat. Nos. 4,312,970 and 4,340,619 and U.S. patentapplication Ser. No. 555,755 filed Nov. 28, 1983, all of which have beenincorporated by reference.

The preceramic silazane polymers described in U.S. Pat. No. 4,312,970are prepared by contacting and reacting in an inert, essentiallyanhydrous, atmosphere, an organochlorosilane or a mixture oforganochlorosilanes of general formula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products. wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; and c has avalue of 1 or 2.

The organochloromonosilanes of U.S. Pat. No. 4,312,970 are those havingthe general formula

    R'.sub.c SiCl.sub.(4-c)

where R' is vinyl or an alkyl radical containing 1-3 carbon atoms or thephenyl group. Thus, those groups which are contemplated as being usefulin this invention are methyl, ethyl, propyl, vinyl, and phenyl. The R'groups can all be the same or they can be different. Theorganochloromonosilanes are common commodity chemicals and arecommercially available and, therefore, an explanation as to theirpreparation does not appear to be necessary. The value of c is 1 or 2.Thus, single organic group substituted silanes such as CH₃ SiCl₃, C₆ H₅SiCl₃, CH₂ ═CHSiCl₃, CH₃ CH₂ SiCl₃ or CH₃ (CH₂)₂ SiCl₃ and doubleorganic substituted silanes such as (CH₃)₂ SiCl₂, (C₂ H₅)₂ SiCl₂ and(CH₂ ═CH) (CH₃)SiCl₂ and mixtures of such silanes, for example CH₃ SiCl₃and (CH₃)₂ SiCl₂, can be used. It is preferred that whenorganochlorosilane mixtures are used, the number of units ofdiorgano-substituted silicon atoms should not exceed the number of unitsof monoorgano-substituted silicon atoms.

The preceramic silazane polymers of U.S. Pat. No. 4,340,619 are preparedby contacting and reacting in an inert, essentially anhydrous,atmosphere, a chlorine-containing disilane or a mixture ofchlorine-containing disilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5 and the sum of (d+e) is equal to three.

The chlorine-containing disilanes of U.S. Pat No. 4,340,619 are thosedisilanes having the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

where R' is vinyl, an alkyl radical containing 1-3 carbon atoms or thephenyl group. Thus, the R' groups are methyl, ethyl, propyl, vinyl andphenyl. The R' groups can all be the same or they can be different. Thechlorine-containing disilanes can be those found in the residue from theDirect Process for producing halosilanes (Eaborn. C., "OrganosiliconCompounds", Butterworth Scientific Publications. London, 1960, pg. 1).The Direct Process is the reaction between silicon metal and aliphatichalides, generally methyl chloride, at elevated temperature in thepresence of catalyst, generally copper, to produce chlorosilanes. Forthe chlorine-containing disilanes described above, the value of d and eis from 0.5-3 and 0-2.5 respectively, and the sum of (d+e) is equal tothree. Examples of chlorine-containing disilanes are (Cl₂ (CH₃)Si)₂,(Cl(CH₃)₂ Si)₂, (Cl₂ C₂ H₅ Si)₂, (Cl(C₆ H₅)₂ Si)₂ and (Cl₂ (CH₂═CH)Si)₂. Monosilanes can also be used in admixtures with the abovedescribed chlorine-containing disilanes. Examples include CH₃ SiCl₃,(CH₃)₂ SiCl₂, H(CH₃)₂ SiCl, (CH₃)₃ SiCl, (CH₂ ═CH)(CH₃)₂ SiCl, (C₂ H₅)₂SiCl₂, C₆ H₅ SiCl₃, as well as (C₆ H₅)₂ SiCl₂, and (C₆ H₅)₃ SiCl. Whenpolysilazane polymers are prepared in accordance with U.S. Pat. No.4,340,619 for use in this invention it is preferred that mixtures ofchlorine-containing disilanes be employed where the number of units ofdiorgano-substituted silicon atoms does not exceed the number of unitsof monoorgano-substituted silicon atoms.

The preceramic silazane polymers of application Ser. No. 555,755 areprepared by contacting and reacting in an inert, essentially anhydrousatmosphere, trichlorosilane with a disilazane at a temperature in therange of 25° to 300° C. while removing byproduced volatile products,wherein said disilazane has the general formula

    (R.sub.3 Si).sub.2 NH

where R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms. It appearsthat some component, possibly a hydrolysis product, in agedtrichlorosilane is detrimental in the preparation of this preceramicsilazane polymer. Such contaminated trichlorosilanes can be suitablypurified by distillation. Other purification methods may also beemployed. It is also preferred that the reactants be added in such amanner that the initial reaction exotherm is kept to a minimum. Onereactant may be added slowly to the other reactant, or the addedreactant may be cooled, or the reaction vessel may be cooled to keep thereaction exotherm low. Other methods or combination of methods may alsobe used. In general, it is preferred that the reaction be controlledsuch that the initial reaction temperature due to the exotherm is lessthan about 50° C., and most preferably, less than 35° C. In general,more reproducible results are obtained when purified trichlorosilane isused and when the initial reaction exotherm is controlled carefully.

The second reactant in U.S. Pat. Nos. 4,312,970, 4,340,619, andapplication Ser. No. 555,755 is a disilazane of the general formula (R₃Si)₂ NH. R in this formula is vinyl, hydrogen, an alkyl radical of 1-3carbon atoms or the phenyl group. Therefore, R, for purposes of thisformula, is represented by hydrogen, methyl, ethyl, propyl, vinyl andphenyl. Each R group in this formula can be the same or they can bedifferent. Examples of the disilazanes include ((CH₃)₃ Si)₂ NH, (C₆ H₅(CH₃)₂ Si)₂ NH, ((C₆ H₅)₂ CH₃ Si)₂ NH, (CH₂ ═CH(CH₃)₂ Si)₂ NH, (CH₂═CH(CH₃)C₆ H₅ Si)₂ NH, (CH₂ ═CH(C₆ H₅)₂ Si)₂ NH, (CH₂ ═CH(C₂ H₅)₂ Si)₂NH, (H(CH₃)₂ Si)₂ NH and (CH₂ ═CH(C₆ H₅)C₂ H₅ Si)₂ NH.

The reactants in U.S. Pat. Nos. 4,312,970, 4,340,619, and applicationSer. No. 555,755 are brought together in an inert, essentially anhydrousatmosphere. By "inert" we mean that the reaction is carried out under ablanket of inert gas, such as argon, nitrogen, or helium. What we meanby "essentially anhydrous" is that the reaction is preferably carriedout in an absolutely anhydrous atmosphere but minute amounts of moisturecan be tolerated.

When the reactants are contacted with each other, as described in U.S.Pat. Nos. 4,312,970 and 4,340,619 and application Ser. No. 555,755 thereaction begins which forms an intermediate amino compound. Uponheating, additional amino compound is formed and upon continued heating.R₃ SiCl is distilled from the reaction mixture and a silazane polymer isformed. The order of addition of the materials does not appear to becritical. As the temperature is raised higher, more condensation takesplace and crosslinking occurs with residual R₃ Si-- that is notdistilled from the mixture acting as a chain stopper. This controlallows one to stop the reaction at any point to obtain almost anydesired viscosity. The desirable temperature range for this reaction is25° to 300° C. A preferred temperature range for this reaction is 125°to 300° C. The length of time that the reaction requires depends on thetemperature employed and the viscosity one wishes to achieve. What ismeant by "volatile products" are the distillable byproduced productsthat are formed by the reactions set forth above. These materials can berepresented by (CH₃)₃ SiCl, (CH₂ ═CH)(C₆ H₅)₂ SiCl, CH₃ (C₆ H₅)₂ SiCl,(CH₃)₂ C₆ H₅ SiCl and (CH₂ ═CH)(CH₃)₂ SiCl. Sometimes, the processrequires the use of a vacuum along with the heat in order to removethese materials from the reaction mixture.

After the preceramic silazane polymer has been rendered infusible bytreatment with steam or a steam and oxygen mixture, the infusiblepreceramic silazane polymer is fired to an elevated temperature of atleast 750° C. in an inert atmosphere, vacuum or ammonia-containingatmosphere until the mixture is converted to a ceramic material.Preferably the pyrolysis temperature is from about 1000° C. to about1600° C. Since the preceramic silazane polymers of this invention havebeen rendered infusible prior to pyrolysis, the pyrolysis step may becarried out by quickly raising the temperature to the desired level. Ifthe preceramic silazane polymer is of sufficient viscosity or if itpossesses a sufficiently low melt temperature, it can be shaped first,then rendered infusible, and then finally pyrolyzed to give a ceramicshaped article such as a fiber. Preferably the preceramic silazanepolymers used in the practice of this invention have a penetrationtemperature of about 50° to 300° C. and most preferably in the range of70° to 200° C. Such a penetration temperature allows for the formationof preceramic silazane fibers by known spinning techniques.

So that those skilled in the art can better appreciate and understandthe invention, the following examples are given. Unless otherwiseindicated, all percentages are by weight. The preceramic polymers werefired to elevated temperature using either an Astro Industries Furnace1000A water cooled graphite resistance heated model 1000.3060-FP- 12, aLindberg tube furnace (Heavy Duty SB Type S4877A) or a ThermolyneF-21100 tube furnace.

Oxygen was determined using LECO analysis.

The penetration temperature of the preceramic silazane polymer wasmeasured with a DuPont Instruments Thermoanalyzer Model 1090 equippedwith a Model 1091 DuPont Disk Memory and a DuPont Model 943Thermomechanical Analyzer. The penetration temperature is related to thesoftening point temperature.

The tensile strength and elastic modulus were determined on a singlefiber employing a computer controlled Instron tester Model 1122 equippedwith pneumatic jaws and a 500 g load cell. Values reported are anaverage of ten individual tests. The procedure used was similar to ASTM3379-75.

EXAMPLE 1

A preceramic silazane polymer, labeled polymer A, was prepared byreacting a mixture of disilanes obtained from the direct process andphenylvinyldichlorosilane with hexamethyldisilazane. The mixture ofdisilanes and phenylvinyldiclorosilane contained 49.1 weight percent Cl₂CH₃ SiSiCH₃ Cl₂, 28.2 weight percent Cl₂ CH₃ SiSi(CH₃)₂ Cl, 6.4 weightpercent Cl(CH₃)₂ SiSi(CH₃)₂ Cl, 0.7 weight percent low boilingimpurities, and 15.4 weight percent (C₆ H₅)(CH₂ ═CH)SiCl₂. Thehexamethyldisilazane was added to the disilane mixture at a levelequivalent to 0.75 moles of N-H present in the hexamethyldisilazane permole of Si--Cl present in the disilane and silane mixture. The resultingmixture was heated at a rate of 1.1° C./min. to 230° C. All reactionsteps were carried out under a nitrogen atmosphere. Volatiles wereremoved by distillation throughout the heating process. The resultingsilazane polymer A had a penetration temperature of about 70° C.Preceramic fibers were prepared by standard spinning techniques at 144°C. by extrusion through a spinneret with 0.01 inch diameter holes. Thepreceramic silazane fibers prepared from polymer A were approximately 30microns in diameter.

Another preceramic silazane polymer, labeled polymer B, was prepared ina similar manner except as noted. The mixture of disilanes andphenylvinyldiclorosilane contained 49.8 weight percent Cl₂ CH₃ SiSiCH₃Cl₂, 27.7 weight percent Cl₂ CH₃ SiSi(CH₃)₂ Cl, 6.7 weight percentCl(CH₃)₂ SiSi(CH₃)₂ Cl, 1.0 weight percent low boiling impurities, and14.7 weight percent (C₆ H₅)(CH₂ ═CH)SiCl₂. The resulting mixture washeated at a rate of 1.1 C/min. to 230° C. and held at 230° C. for 30minutes. The resulting polymer B had a penetration temperature of about136° C. Preceramic fibers were prepared by standard spinning techniquesat 219° C. The preceramic silazane fibers prepared from polymer B wereapproximately 15 microns in diameter.

A small sample (0.1-0.5 g) of fibers was placed in a glass tube insertedin a tube furnace. The fibers were cured or rendered infusible byexposure to humidified air at various temperatures for varying lengthsof time. The humidified air was prepared by bubbling air through liquidwater at room temperature before passage over the fibers. The humidifiedair was at about 100 percent relative humidity at room temperature.After treatment with humidified air the fibers were pyrolyzed in anargon atmosphere by heating the fibers at a rate of 3° C./min. to 1200°C. The results are presented in Table I. For runs where only onetemperature is given, the fibers were exposed to the humidified air at aconstant temperature for the listed amount of time. Where a temperaturerange is given, the fibers were exposed to the humidified air by heatingfrom the lower to higher temperature at a rate of 3.75° C./min and thenholding at the higher temperature for the remainder of the specifiedtime. None of the fibers treated with humidified air under theconditions indicated in Table I melted or fused together upon pyrolysisto 1200° C. Preceramic fibers which were not cured or rendered infusibledid fuse together upon pyrolysis to 1200° C.

                  TABLE I                                                         ______________________________________                                        CURE CONDITIONS    CERAMIC FIBERS                                             POLY-  TEMP      TIME      OXYGEN   CERAMIC                                   MER    (°C.)                                                                            (hrs)     (%)      YIELD (%)                                 ______________________________________                                        A      65        17.0      10.8     74.3                                      B      65-150    22.5      24.0     74.0                                      B      65-175    31.0      27.0     75.0                                      B      65-200    36.0      30.0     77.0                                      ______________________________________                                    

EXAMPLE 2

Another preceramic silazane polymer was prepared using the proceduresoutlined in U.S. patent application Ser. No. 555,755 (filed Nov. 28,1983). The preceramic polymer was prepared by mixing one equivalent oftrichlorosilane with 2.25 equivalents of hexamethyldislazane at 0-15° C.The mixture was held at room temperature overnight and then heated at arate of 1° C./min. to 250° C. The reaction mixture was held at 250° C.for one hour. All reaction steps were carried out under an argonatmosphere. During the heating period, volatiles were removed bydistillation. The preceramic polymer was cooled, dissolved in toluene,filtered through a 0.45 micron membrane, and then strip distilled at250° C. under vacuum in a molecular still. The resulting preceramicpolymer had a penetration temperature of 76° C. The preceramic polymerwas spun into fibers using a melt rheometer with a single 0.02 inchorifice at a temperature of 154° C. The preceramic fibers had diametersof 40-50 microns.

One sample of these preceramic fibers was fired in a 100 volume percentammonia atmosphere. The fibers were not cured prior to the ammoniapyrolysis. After the pyrolysis to 1200° C. the fiber had melted andfused together.

Another sample of these preceramic fibers was cured by exposure tohumidified air. The fiber were exposed to humidified air (about 8 cfhflow and 100 percent relative humidity at room temperature) where thetemperature was raised from 35° to 165° C. at a rate of 2.7° C./hr.

Both cured and uncured fiber samples were pyrolyzed to 1200° C. in a 100volume percent ammonia atmosphere. Samples with no cure melted and fusedduring pyrolysis to 1200° C. Cured fibers did not melt or fuse whenpyrolyzed to 1200° C. The results are given in Table II. Uncuredpreceramic fibers gave a white, fused, ceramic mass with a ceramic yieldof 50.7 weight percent. Cured preceramic fibers gave white ceramicfibers with a ceramic yield of 63.6 weight percent which contained 31.4weight percent nitrogen and about 0.1 weight percent carbon and whichhad a tensile strength of 26 MPa and an elastic modulus of 6.9 GPa.

EXAMPLE 3

Preceramic silazane fibers similar to those used in Example 2 were usedin this example. The fibers were treated with humidified air containingapproximately 100 percent relative humidity at room temperature usingthe procedure of Example 1. The cured fibers were then pyrolyzed in anargon atmosphere to 1200° C. as in Example 1. The results are presentedin Table II. None of the fibers treated with humidified air under theconditions indicated in Table II melted or fused together upon pyrolysisto 1200° C. Preceramic fibers which were not cured or rendered infusibledid fuse together upon pyrolysis to 1200° C.

                  TABLE II                                                        ______________________________________                                                       CERAMIC FIBERS                                                 CURE CONDITIONS              CERAMIC                                          TEMP (°C.)                                                                       TIME (hrs) OXYGEN (%)  YIELD (%)                                    ______________________________________                                        35-100    48         3.0         58.6                                         35-120    22         1.2         --                                           35-155    32         2.1         --                                           35-165    48         17.8        64.0                                         35-211    47         10.9        --                                           ______________________________________                                    

EXAMPLE 4

A preceramic silazane polymer was prepared by reacting a mixture ofdisilanes obtained from the direct process, methylvinyldichlorosilane,and methylhydrogendichlorosilane with hexamethyldisilazane. The mixtureof disilanes, methylvinyldichlorosilane, and methylhydrogendichlorsilanecontained 50.0 weight percent Cl₂ CH₃ SiSiCh₃ Cl₂, 36.6 weight percentCl₂ CH₃ SiSi(CH₃)₂ Cl, 2.9 weight percent Cl(CH₃)₂ SiSi(CH₃)₂ Cl, 5.8weight percent (CH₃)(CH₂ ═CH)SiCl₂, and 4.7 weight percent CH₃ (H)SiCl₂.The hexamethyldisilazane was added to the disilane and silane mixture ata level equivalent to 0.75 moles of N-H present in thehexamethyldisilazane per mole of Si--Cl present in the disilane andsilane mixture. The resulting mixture was heated at a rate of 2.5°C./min. to 80° C. and held at 80° C. for 20 minutes. The temperature wasthen raised to 220° C. at a rate of 1.0° C./min. and held at 220° C. for10 minutes. All reaction steps were carried out under an argonatmosphere. Volatiles were removed throughout the heating process. Thispreceramic silazane polymer had a penetration temperature of 112° C.Fibers were prepared from this material with an average diameter ofabout 40 microns.

The preceramic silazane fibers were exposed to steam or steam and oxygenatmosphere in the same tube furnace used for the later pyrolysis step.The preceramic fibers were placed in a boat in the center of the tubefurnace. The system was first flushed with argon. Then the desired gaswas passed over the sample boat at the desired temperature. Steam wasgenerated by passage of the desired gas over liquid water prior toentering the tube furnace. The amount of steam in the gas phase wascontrolled by varying the temperature of the liquid water. Thetemperature of the liquid water and the preceramic material beingtreated were controlled independently. After completion of the steam orsteam and oxygen treatment, the system was purged with argon and thetemperature of the tube furnace increased to the desired pyrolysistemperature under an argon atmosphere.

Two techniques were employed to gauge the effectiveness of the steam orsteam and oxygen mixture treatment step in rendering the preceramicsilazane fibers infusible. A simple solubility test was used to estimatethe effectiveness of the treatment step of this invention. A smallamount of the fibers was placed in toluene at room temperature and thedegree of solubility was observed. Uncured material was essentiallycompletely soluble in toluene whereas material which had been renderedinfusible was essentially insoluble in toluene. The second method toevaluate the degree of cure or degree to which the preceramic silazanepolymer had been rendered infusible was actual pyrolysis. Pyrolysis is adirect test to measure the effectiveness of the treatment. If the fibersdo not fuse together, the treatment was sufficient to render thematerial or fibers infusible. Two basic pyrolysis schedules wereemployed. With "slow pyrolysis", preceramic fibers were heated at 5° to13° C./min. to 1200° C. in argon. With "fast pyrolysis", preceramicfibers were heated at 100° C./min. to 600° C. in argon. Fibers which didnot fuse during the pyrolysis treatment were considered to have beenrendered infusible by the steam or steam and oxygen mixture treatmentand were rated "pass". Fibers which were uncured failed both the slowand fast pyrolysis tests. The results are presented in Table III.

                  TABLE III                                                       ______________________________________                                        CURE                                                                          CONDITIONS                                                                    TEMP        TIME                PYROLYSIS                                     GAS    (°C.)                                                                           (min.)  SOLUBILITY                                                                              SLOW   FAST                                 ______________________________________                                        O.sub.2 /H.sub.2 O                                                                   152      1       slightly  pass   pass                                                         soluble                                               O.sub.2 /H.sub.2 O                                                                   155      2       very slightly                                                                           pass   pass                                                         soluble                                               O.sub.2 /H.sub.2 O                                                                   160      3       insoluble pass   pass                                 argon/ 158      1       mostly    pass   pass                                 H.sub.2 O               insoluble                                             ______________________________________                                    

That which is claimed is:
 1. A method of preparing a preceramicmaterial, which method comprises (1) treating a preceramic silazanepolymer with steam at a temperature in the range of about 35° C. to 200°C. for a time sufficient to render the preceramic silazane polymerinfusible wherein the treatment temperature is sufficiently low so thatthe preceramic silazane polymer remains unfused during the treatmentstep.
 2. A method as defined in claim 1 wherein said preceramic silazanepolymer is treated with steam at a temperature below the penetrationtemperature of said preceramic silazane polymer and wherein saidpreceramic silazane polymer is prepared by contacting and reacting in aninert, essentially anhydrous, atmosphere, an organochlorosilane or amixture of organochlorosilanes of the general formula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; and c has avalue of 1 or
 2. 3. A method as defined in claim 1 wherein saidpreceramic silazane polymer is treated with steam at a temperature belowthe penetration temperature of said preceramic silazane polymer andwherein said preceramic silazane polymer is prepared by contacting andreacting in an inert, essentially anhydrous, atmosphere, achlorine-containing disilane or a mixture of chlorine-containingdisilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5 and the sum of (d+e) is equal to three.4. A method as defined in claim 1 wherein said preceramic silazanepolymer is treated with steam at a temperature below the penetrationtemperature of said preceramic silazane polymer and wherein saidpreceramic silazane polymer is prepared by contacting and reacting in aninert, essentially anhydrous atmosphere, trichlorosilane with adisilazane at a temperature in the range of 25° C. to 300° C. whileremoving byproduced volatile products, wherien said disilazane has thegeneral formula

    (R.sub.3 Si).sub.2 NH

where R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms.
 5. A methodas defined in claim 1 wherein said preceramic silazane polymer istreated with steam at a temperature below the penetration temperature ofsaid preceramic silazane polymer and wherein said preceramic silazanepolymer is prepared by contacting and reacting in an inert, essentiallyanhydrous, atmosphere, a mixture of (1) chlorine-containing disilane ora mixture of chlorine-containing disilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

and (2) an organochlorosilane or a mixture of organochlorosilanes of thegeneral formula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5; the sum of (d+e) is equal to three;and c has a value of 1 or
 2. 6. A method as defined in claim 1 whereinsaid preceramic polymer is treated with steam in the temperature rangeof 35° C. to 10° C. below the penetration temperature of said preceramicsilazane polymer.
 7. A method as defined in claim 2 wherein saidpreceramic polymer is treated with steam in the temperature range of 35°C. to 10° C. below the penetration temperature of said preceramicsilazane polymer.
 8. A method as defined in claim 3 wherein saidpreceramic polymer is treated with steam in the temperature range of 35°C. to 10° C. below the penetration temperature of said preceramicsilazane polymer.
 9. A method as defined in claim 4 wherein saidpreceramic polymer is treated with steam in the temperature range of 35°C. to 10° C. below the penetration temperature of said preceramicsilazane polymer.
 10. A method as defined in claim 5 herein saidpreceramic polymer is treated with steam in the temperature range of 35°C. to 10° C. below the penetration temperature of said preceramicsilazane polymer.
 11. A method of preparing a preceramic material, whichmethod comprises (1) treating a preceramic silazane polymer with a steamand oxygen mixture at a temperature in the range of about 35° C. to 200°C. for a time sufficient to render the preceramic silazane polymerinfusible wherein the treatment temperature is sufficiently low so thatthe preceramic silazane polymer remains unfused during the treatmentstep.
 12. A method as defined in claim 11 wherein said preceramicsilazane polymer is treated with said steam and oxygen mixture at atemperatue below the penetration temperature of said preceramic silazanepolymer and said preceramic silazane polymer is prepared by contactingand reacting in an inert, essentially anhydrous, atmosphere, anorganochlorosilane or a mixture of organochlorosilanes of the generalformula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperatue in the range of 25° C. to 300° C. while distillingbyproduced volatile prodcuts, wherein R' is selected form the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; and c has avalue of 1 or
 2. 13. A method as defined in claim 11 wherein saidpreceramic silazane polymer is treated with said steam and oxygenmixture at a temperatue below the penetration temperature of saidpreceramic silazane polymer and said preceramic silazane polymer isprepared by contacting and reacting in an inert, essentially anhydrous,atmosphere, a chlorine-containing disilane or a mixture ofchlorine-containing disilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containaing 1 to 3carbon atoms; R is selected from the group consisting of vinyl,hydrogen, phenyl, and alkyl radicals containing 1 to 3 carbon atoms; dhas a value of 0.5-3; e has a value of 0-2.5 and the sum of (d+e) isequal to three.
 14. A method as defined in claim 11 wherein saidpreceramic silazane polymer is treated with said steam and oxygenmixture at a temperature below the penetration temperature of saidpreceramic silazane polymer and said preceramic silazane polymer isprepared by contacting and reacting in an inert, essentially anhydrousatmosphere, trichlorosilane with a disilazane at a temperature in therange of 25° C. to 300° C. while removing byproduced volatile products,wherein said disilazane ahs the general formula

    (R.sub.3 Si).sub.2 NH

where R is selected form the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms.
 15. A methodas defined in claim 11 wherein said preceramic silazane polymer istreated with said steam and oxygen mixture at a temperature below thepenetration temperature of said preceramic silazane polymer and saidpreceramic silazane polymer is prepared b y contacting and reacting inan inert, essentially anhydrous, atmosphere, a mixture of (1)chlorine-containing disilane or a mixture of chlorine-containingdisilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

and (2) an organochlorosilane or a mixture of organochlorosilanes of thegneral formula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5; the sum of (d+e) is equal to three;and c has a vlaue of 1 or
 2. 16. A method as defined in claim 11 whereinsaid oxygen in said steam and oxygen mixture is in the form of air. 17.A method as defined in claim 12 wherein said oxygen in said steam andoxygen mixture is in the form of air.
 18. A method as defined in claim13 wherein said oxygen in said steam and oxygen mixture is in the formof air.
 19. A method as defined in claim 14 wherein said oxygen in saidsteam and oxygen mixture is in the form of air.
 20. A method as definedin claim 15 wherein said oxygen in said steam and oxygen mixture is inthe form of air.
 21. A method as defined in claim 16 wherein saidpreceramic polymer is treated with said steam and oxygen mixture in thetemperature range of 35° C. to 10° C. below the penetration temperatureof said preceramic silazane polymer.
 22. A method as defined in claim 17wherein said preceramic polymer is treated with said steam and oxygenmixture in the temperature range of 35° C. to 10° C. below thepenetration temperature of said preceramic silazane polymer.
 23. Amethod as defined in claim 18 wherein said preceramic polymer is treatedwith said steam and oxygen mixture in the temperature range of 35° C. to10° C. below the penetration temperature of said preceramic silazanepolymer.
 24. A method as defined in claim 19 wherein said preceramicpolymer is treated with said steam and oxygen mixture in the temperaturerange of 35° C. to 10° C. below the penetration temperature of saidpreceramic silazane polymer.
 25. A method as defined in claim 20 whereinsaid preceramic polymer is treated with said steam and oxygen mixture inthe temperature range of 35° C. to 10° C. below the penetrationtemperature of said preceramic silazane polymer.
 26. A method as definedin claim 21 wherein the relative humidity of said steam and oxygenmixture at about room temperature is about 100 percent.
 27. A method asdefined in claim 22 wherein the relative humidity of said steam andoxygen mixture at about room temperature is about 100 percent.
 28. Amethod as defined in claim 23 wherein the relative humidity of saidsteam and oxygen mixture at about room temperature is about 100 percent.29. A method as defined in claim 24 wherein the relative humidity ofsaid steam and oxygen mixture at about room temperature is about 100percent.
 30. A method as defined in claim 25 wherein the relativehumidity of said steam and oxygen mixture at about room temperature isabout 100 percent.
 31. Preceramic fibers prepared by the methodcomprising the steps of (1) preparing a preceramic silazane polymer, (2)preparing preceramic fibers from said preceramic silazane polymer, (3)treating the preceramic fibers prepared in step (2) with steam or asteam and oxygen mixture at a temperature in the range of about 35° C.to 200° C. for a time sufficient to render the preceramic fibersinfusible wherein the treatment temperature is sufficiently low so thatthe preceramic silazane fibers remain unfused during the treatment step.32. Preceramic fibers as defined in claim 31 wherein said preceramicsilazane polymer is treated with steam or steam and oxygen mixture at atemperature below the penetration temperature of said preceramicsilazane polymer and said preceramic silazane polymer is prepared bycontacting and reacting in an inert, essentially anhydrous, atmosphere,an organochlorosilane or a mixture of organochlorosilanes of the generalformula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group oonsisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; and c has avalue of 1 or
 2. 33. Preceramic fibers as defined in claim 31 whereinsaid preceramic silazane polymer is treated with steam or steam andoxygen mixture at a temperature below the penetration temperature ofsaid preceramic silazane polymer and said preceramic silazane polymer isprepared by contacting and reacting in an inert, essentially anhydrous,atmosphere, a chlorine-containing disilane or a mixture ofchlorine-containing disilanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected form the groupconsisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5 and the sum of (d+e) is equal to three.34. Preceramic fibers as defined in claim 31 wherein said preceramicsilazane polymer is treated with steam or steam and oxygen mixture at atemperature below the penetration temperature of said preceramicsilazane polymer and said preceramic silazane polymer is prepared bycontacting and reacting in an inert, essentially anhydrous atmosphere,trichlorosilane with a disilazane at a temperature in the range of 25°C. to 300° C. while removing byproducued volatile products, wherein saiddisilazane has the general formula

    (R.sub.3 Si).sub.2 NH

where R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms. 35.Preceramic fibers as defined in claim 31 wherein said preceramicsilazane polymer is treated with steam or steam and oxygen mixture at atemperature below the penetration temperature of said preceramicsilazane polymer and said preceramic silazane polymer prepared bycontacting and reacting in an inert, essentially anhydrous, atmosphere,a mixture of (1) chlorine-containing disilane or a mixture ofchlorine-containing dislanes, of the general formula

    (Cl.sub.d R'.sub.e Si).sub.2

and (2) an organochlorosilane or a mixture of organochlorosilanes of thegeneral formula

    R'.sub.c SiCl.sub.(4-c)

with a disilazane having the general formula

    (R.sub.3 Si).sub.2 NH

at a temperature in the range of 25° C. to 300° C. while distillingbyproduced volatile products, wherein R' is selected from the groupconsisting of vinyl, phenyl, and alkyl raidcals containing 1 to 3 carbonatoms; R is selected from the group consisting of vinyl, hydrogen,phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a valueof 0.5-3; e has a value of 0-2.5; the sum of (d+e) is equal to three;and c has a value of 1 or 2.